Since the hypothesis that polycyclic aromatic hidrocarbons (PAHs) are the
carriers of
the unidentified infrared bands [1]}, [2]},
many efforts have been devoted to understand the
chemical processes
leading to the formation of these molecular species (see, e.g., [3]}).
Circumstellar envelopes around carbon-rich Asymptotic Giant Branch (AGB) stars have been suggested
as the factories of PAHs [4]}.
The detection of benzene in the carbon-rich protoplanetary nebula CRL 618 [5]}
suggests a bottom-up approach in which the small hydrocarbons formed during the AGB phase, such
as C\(_2\) H\(_2\)  and C\(_2\) H\(_4\) , interact with the ultraviolet (UV) radiation produced by the star
in its evolution to the white
dwarf phase [6]}, [7]}. Other hypotheses involve the proccesing of dust
grains around evolved stars, either through UV photons [8]} or by chemical
processes [9]}.
Hence, it has been surprising to see that cyanide derivatives of PAHs have been found in a
cold prestellar core such as TMC-1, which is well protected against UV radiation
[10]}, [11]}. It is unlikely that these PAH-cyanides arise from a reservoir of
PAHs existing since the early stages of the cloud, since these relatively
small PAHs would not have survived the diffuse cloud stage.
Although [11]} propose
a reasonable chemical network starting with the phenyl radical that could
explain the observed abundances of
cyanonaphthalene, the chemical routes leading to benzene itself are still
unclear. An in situ formation mechanism for benzene must involve abundant
hydrocarbons containing from 2 to 4 carbon atoms. Moreover, some of these
species have to permit an easy cyclization in 2-3 steps to have an efficient yield of benzene or phenyl radical.
The propargyl radical, CH\(_2\) CCH, has been found recently in
TMC-1 by [13]} with an abundance close to 10\(^{-8}\)  relative to H\(_2\) . In addition, complex hydrocarbons
such as vinyl and allenyl acetylene have been also observed with very large abundances [14]}, [15]}. These hydrocarbons may hold the key
to form the first aromatic ring, from which larger PAHs can grow.
